Malaria is one of the most devastating parasitic diseases affecting humans. Indeed, 41% of the world's population lives in areas where malaria is transmitted (e.g., parts of Africa, Asia, the Middle East, Central and South America, Hispaniola, and Oceania). The World Health Organization (WHO) and the Centers for Disease Control (CDC) estimate that malaria infects 300-500 million people and kills 700,000-3 million people annually, with the majority of deaths occurring in children in sub-Saharan Africa. Malaria also is a major health concern to U.S. military personnel deployed to tropical regions of the world. For example, in August 2003, 28% of the 26th Marine Expeditionary Unit and Joint Task Force briefly deployed to Monrovia, Liberia, were infected with the malaria parasite Plasmodium falciparum. In addition, one 157-man Marine Expeditionary Unit sustained a 44% malaria casualty rate over a 12-day period while stationed at Robert International Airport in Monrovia. In all conflicts during the past century conducted in malaria endemic areas, malaria has been the leading cause of casualties, exceeding enemy-inflicted casualties in its impact on “person-days” lost from duty.
To combat malaria during U.S. military operations, preventive drugs, insect repellants, and barriers have been used with some success, but developing drug resistance by the malaria parasite and insecticide resistance by mosquito vectors has limited the efficacy of these agents. Moreover, the logistical burden and side effects associated with the use of these agents often is associated with high non-compliance rates. Vaccines are the most cost effective and efficient therapeutic interventions for infectious diseases. In this regard, vaccination has the advantage of administration prior to military deployment and likely reduction in non-compliance risks. However, decades of research and development directed to a malaria vaccine have not proven successful. Recent efforts have focused on developing vaccines against several specific malaria genes and delivery vector systems including adenovirus, poxvirus, and plasmids. The current status of malaria vaccine development and clinical trials is reviewed in, for example, Graves and Gelband, Cochrane Database Syst. Rev., 1: CD000129 (2003), Moore et al., Lancet Infect. Dis., 2: 737-743 (2002), Carvalho et al., Scand. J. Immunol., 56: 327-343 (2002), Moorthy and Hill, Br. Med. Bull., 62: 59-72 (2002), Greenwood and Alonso, Chem. Immunol., 80: 366-395 (2002), and Richie and Saul, Nature, 415: 694-701 (2002).
Over the past 15-20 years, a series of Phase 1/2 vaccine trials have been reported using synthetic peptides or recombinant proteins based on malarial antigens. Approximately 40 trials were reported as of 1998 (see Engers and Godal, Parisitology Today, 14: 56-64 (1998)). Most of these trials have been directed against the sporozoite stage or liver stage of the Plasmodium life cycle, where the use of experimental mosquito challenges allows rapid progress through Phase 1 to Phase 2a preliminary efficacy studies. Anti-sporozoite vaccines tested include completely synthetic peptides, conjugates of synthetic peptide with proteins such as tetanus toxoid (to provide T cell help), recombinant malaria proteins, particle-forming recombinant chimeric constructs, recombinant viruses, and bacteria and DNA vaccines. Several trials of asexual blood stage vaccines have used either synthetic peptide conjugates or recombinant proteins. There also has been a single trial of a transmission blocking vaccine (recombinant Pfs25). A recurring problem identified in all of these vaccination strategies is the difficulty in obtaining a sufficiently strong and long lasting immune response in humans, despite the strong immunogenic response in animal models.
To overcome these limitations, the development of potent immune-stimulatory conjugates or adjuvants to boost the human response has been explored, in addition to the development of vaccines directed against the circumsporozoite protein (CSP), which is the principal sporozoite coat protein. Anti-CSP vaccines using recombinant proteins, peptide conjugates, recombinant protein conjugates, and chimeric proteins have been shown to elicit anti-CSP antibodies. Although considerable efforts are still being directed at the development of protein-based vaccines, alternative technologies such as DNA and viral based vaccines have shown some promise with regard to immunogenicity and protective efficacy, at least in animal models.
In this regard, DNA vaccines encoding Plasmodium antigens have been developed and can induce CD8+ CTL and IFN-γ responses, as well as protection against sporozoite challenge in mice (see Sedegah et al., Proc. Natl. Acad. Sci. USA, 91: 9866-9870 (1994), and Doolan et al., J. Exp. Med., 183: 1739-1746 (1996)) and monkeys (Wang et al., Science, 282: 476-480 (1998), Rogers et al., Infect. Immun., 69: 5565-5572 (2001), and Rogers et al., Infect. Immun., 70: 4329-4335 (2002)). Furthermore, Phase I and Phase 2a clinical trials have established the safety, tolerability, and immunogenicity of DNA vaccines encoding malaria antigens in normal healthy humans (see, e.g., Wang et al., Infect Immun., 66: 4193-41202 (1998), Le et al., Vaccine, 18: 1893-1901 (2000), and Epstein et al., Hum. Gene Ther., 13: 1551-1560 (2002)). However, the immunogenicity of first and second-generation DNA vaccines in nonhuman primates and in humans has been suboptimal. Even in murine models, DNA vaccines are not effective at activating both arms of the immune system (see, e.g., Doolan et al., supra, Sedegah et al., supra, Sedegah et al., Proc. Natl. Acad. Sci. USA, 95: 7648-7653 (1998), Zavala et al., Virology, 280: 155-159 (2001), and Pardoll, Nat. Rev. Immunol, 2: 227-238 (2002)).
Thus, there remains a need for improved methods that effectively deliver malaria antigens to human hosts so as to prevent the onset of disease and/or protect human hosts from further infections. The invention provides such methods. This and other advantages of the invention will become apparent from the detailed description provided herein.